The present invention relates generally to dampers for lightweight mechanical assemblies and more particularly to a damper for a mechanical shutter such as a photographic shutter.
Mechanical assemblies having moving parts frequently require damping to protect the parts from damaging impacts and to increase the life and reliability of the assemblies. This is particularly true where the mechanical assembly is lightweight and the impact is repetitive and at a high rate of speed. Electrically operated lens shutters used in various types of photographic and laboratory equipment meet these criteria. For example, a shutter can open and close in a fraction of a second. Electronically operated shutters can operate at frequencies of 2 to 400 cycles per second and higher.
Lens shutters generally are of two types. In one type, a so called xe2x80x9cguillotinexe2x80x9d shutter has one or two thin, metal blades or leaves arranged to cover a lens opening. Pivot connections allow each blade to swing between a closed position where the blades cover the lens opening and an open position where the blades are drawn aside from the lens opening.
In a second type, a plurality of pivotally mounted blades, usually five, are arranged around the lens. Each blade is connected to a rotatable ring. In the operation of these rotary shutters, the rotation of the ring in one direction causes the blades to swing in unison to an open position. Counter rotation of the ring swings the blades to a closed position over the lens opening after exposure.
It is common in the first type of shutter to provide a shock absorber or damper that absorbs the impact as the blades are pivoted between the open and closed positions. In this respect, reference is made to U.S. Pat. No. 3,595,553 and U.S. Pat. No. 3,664,251, the disclosures of which are incorporated herein by reference. As disclosed in these references, the shock absorber operates to stop the shutter blade very rapidly, yet softly without damage and with little or no bounce. To Applicant""s knowledge, a comparable shock absorbing system has not been used in connection with the rotating ring type of shutters.
One material known to have damping characteristics suitable for use in the damping system of photographic shutters is a ISOLOSS(copyright) high density molded product made by Aearo Company. The ISOLOSS(copyright) products are high density damped polyurethane thermoset polymers. Sheets and molded parts of this material have been used in a variety of applications relating to noise, shock and vibration reduction. The material has favorable hysteretic damping properties, good compliance and is able to absorb and store mechanical energy while effectively dissipating it in the form of heat.
Dampers made from this polyurethane molded material have been used for some time in combination with other materials such as metals and Teflon coated metal and have provided good results in some applications. However, one disadvantage of the polyurethane molded shock absorbing formulations is that they have a rubbery characteristic and tend to stick to the surfaces that they impact while performing the damping function. In an attempt to remedy this, the impact surface often is coated with Teflon. A Teflon coated surface can reduce or eliminate the sticking when contacting the polyurethane molded damper, however, the Teflon coated surface introduces other problems. Teflon tends to scale and form a powdery residue somewhat like talcum powder in its consistency. In many applications the residue is not harmful but, in some applications, such as optical shutters, the residue may find its way to the surfaces of optical components such as lenses. This powder on the surface of the lens is detrimental to the operation of the lens.
Accordingly, it is an object of the present invention to provide an improved damping system for photographic shutters and in particular, high speed rotary shutters.
Another object of the present invention is to provide a damping system employing a polyurethane molded damper that eliminates sticking of the damper to the impact surface.
A further object is to provide a damping system having an increased operational life.
In the present invention a rotary shutter is provided with a damping system wherein the swinging motion of the shutter drive arm through an arcuate path of travel is arrested by a detent positioned to engage a member carried by the drive arm. The member is a beam that is generally triangular in cross section. Fixed to each side of the beam is a damper formed of a highly damped polyurethane thermoset material. Impact of the dampers against the detent arrests the movement of the drive arm.
The impact surface of the detents comprises a bumper composed of an ultra high molecular weight polyethylene. This material, together with the shape and disposition of the bumper and damper provide a limited contact area. Having a limited contact area concentrates the impact energy and improves the efficiency of the damping system. When the damper is struck, the material at the point of impact undergoes a transformation from a glass phase to a rubber phase and it is the rubber phase that tends to stick to the impact surface. By limiting the area of contact to a line or more preferably a point contact, the potential xe2x80x9cstickyxe2x80x9d area is reduced to a minimum.
Accordingly, by careful selection of the shape, materials and arrangement of the damper and bumper, sticking is virtually eliminated. Life testing demonstrates that the components have a much greater life than heretofore achieved with other components. Damping characteristics of the combination are close to ideal for electromechanical shutters and likely for other applications in that the design essentially eliminates bounce or rebound after impact.
Accordingly, the present invention may be characterized in one aspect thereof by a damping system for arresting motion of a small mass such as a photographic shutter or the like driven by a drive arm mounted for swinging motion through an arcuate path of travel. The damping system includes a detent having opposite ends arranged to arrest motion of the arm at each end of the path of travel. In this respect the opposite ends of the detent carry bumpers formed of a high molecular weight polyethylene that are disposed to impact a damper on the drive arm, the damper being composed of a high density polyurethane thermoset material. Both the bumper and damper are shaped to provide substantially point contact during the duration of the engagement between the damper and the bumper.